Fix: Correctly flag always-truthy exported and imported enum conditions (ts2845)

[iapoorv01]

🚀 Problem Statement

Fixes microsoft/TypeScript#63565.

Currently, the control flow analysis incorrectly suppresses the ts2845 error ("This condition will always return 'true'") for known truthy enum conditions if the enum is declared with the export modifier, or if it is imported from another module as an alias. For instance, testing AdapterOutputType.PAGES === 1 will fail to emit the error if AdapterOutputType is an exported enum.

🕵️ Root Cause Analysis

When testing truthiness in checkTestingKnownTruthyType, the compiler verifies whether the referenced expression belongs to an enum by checking SymbolFlagsEnum. However, the current logic only checks the resolved local symbol (c.getResolvedSymbolOrNil(location.Expression())).

This check inadvertently drops the SymbolFlagsEnum tag in two key scenarios:

1. Exported Enums: The local symbol is bound strictly with SymbolFlagsExportValue. The actual SymbolFlagsEnum tag is stored in the underlying .ExportSymbol.
2. Imported Enums: The resolved symbol acts as an alias (SymbolFlagsAlias), completely masking the underlying enum flags.

As a result, Flags & ast.SymbolFlagsEnum evaluates to 0, the compiler silently assumes it is an open-ended generic property access, and the truthiness validation is pessimistically bypassed.

🛠️ The Fix

This PR modifies internal/checker/checker.go -> checkTestingKnownTruthyType to properly traverse and unwrap the symbol tree before performing the SymbolFlagsEnum verification:

1. Evaluates and resolves any SymbolFlagsAlias (covering cross-module imports).
2. Unwraps SymbolFlagsExportValue via getExportSymbolOfValueSymbolIfExported to reach the true declaration flags.

📊 Pros & Cons

• Pros:
  • Restores strict type-checking and control flow parity for ts2845 across both local and exported enums without overhauling binding mechanisms.
  • Re-uses existing, battle-tested alias-resolution internal helpers (resolveAlias and getExportSymbolOfValueSymbolIfExported).
• Cons:
  • Negligible overhead (a single recursive alias unwrap) added to truthiness evaluations on logical conditions.

🧪 Testing

• Verified logic locally against the checkTestingKnownTruthyType control flow pipeline.
• Existing tests and baselines pass (no unintentional regressions to standard AST handling).
• Note: A dedicated baseline for this specific issue has been deferred, as generating .types and .errors baselines requires go to execute npx hereby baseline-accept natively.

[Copilot]

Pull request overview

Adjusts checker control-flow analysis in the TypeScript native port to correctly detect always-truthy enum-member conditions (TS2845) when the enum is exported or referenced through an import alias.

Changes:

• Unwraps alias symbols (SymbolFlagsAlias) before checking enum-ness for TS2845 suppression logic.
• Unwraps exported value symbols (SymbolFlagsExportValue via getExportSymbolOfValueSymbolIfExported) to access underlying enum flags.

[iapoorv01]

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